Generally, aluminum or aluminum alloys are protected by forming thereon an intermediate corrosion resistant conversion coating and then painting over the corrosion resistant coating. Therefore, the corrosion resistant coating must be intimately bonded to the aluminum surface and also provide the required adhesion with the desired final aluminum coating--i.e., paint.
An accepted process for protecting aluminum and aluminum alloys with a corrosion resistant intermediate coating is to coat the surface of the aluminum and aluminum alloys with a protective conversion coating of an acid based hexavalent chromium composition.
Hexavalent chromium has been widely accepted as an intermediate corrosion resistant conversion coating because it protects the aluminum and aluminum alloy surfaces for extended periods of time. The hexavalent chromium provides a corrosion resistant coating which can withstand a salt fog bath for more than 168 hours. The coated aluminum or aluminum alloy is placed in a salt fog at 95.degree. F. according to ASTM method B-117 for at least 168 hours and then removed. This requirement is necessary for many applications. Further, the hexavalent chromium composition provides an intermediate coating which is receptive to the application and retention of other coatings, such as paints, to the aluminum or aluminum alloy surfaces.
The excellent features of the hexavalent chromium composition have made these compositions used extensively for the corrosion resistant protection of aluminum and aluminum alloys and as an intermediate corrosion resistant coating.
However, the hexavalent chromium compositions have a serious side effect. Chromium is highly toxic and the spent chromium compositions provide an ecological problem. Many people in the industry are attempting to eliminate this ecologically damaging waste problem and it is very costly.
Other corrosion resistant compositions have been suggested but they have not been as successful as the hexavalent chromium compositions.
In 1940 Collari reported for the first time on the inhibiting action of potassium permanganate against attack by sodium hydroxide on aluminum. (Chemical Abstracts 5814-6, Volume 34, 1940) In 1941 Lilli Reschke and Heinrick Neunzig (Chemical Abstracts, Vol. 36, 1942, 5760-5-7) reported the first study on the inhibiting action of potassium permanganate against the attack by sodium hydroxide on aluminum. Finally, in 1947 (Chemical Abstracts 4759 e.g., Vol. 41, 1947) Collari and Fongi also compared the inhibiting action of potassium permanganate to sodium chromate in inhibiting attack by sodium hydroxide on aluminum at various temperatures.
Various compositions of sodium chromate and sodium hydroxide were utilized, and sheets of aluminum were immersed in these solutions. The solutions all had a pH of 12.5 or greater than 12.5.
It was appreciated, after these articles, that the most effective corrosion resistant coatings were those which are acid based. The basic compositions of hexavalent chromium were not effective for prolonged corrosion protection of aluminum surfaces. Neither the basic chromium nor the basic permanganate which have a pH of greater than 12.5, would be appropriate for the corrosion resistant coating of aluminum wherein the aluminum requires a corrosion protection in a salt fog of greater than 168 hours. Further, the industry decided that basic compositions were inadequate for their purposes because highly basic solutions attacked aluminum surfaces. The industry has concentrated their efforts on acid based conversion coating compositions.
In some applications, the acid chromate composition was combined with potassium permanganate to form a black coating. The pH of the solution stayed in the preferred range of 2-3, U.S. Pat. No. 4,145,234.
Also, it has been suggested that the use of the oxidizing agents, sodium or potassium chromate and potassium permanganate, may be added to an electrolyte solution to inhibit the corrosion of aluminum electrodes.
In the immersion coating of aluminum with a chromium coating the thickness of the chromium coating is usually varied by the amount of time the aluminum or aluminum alloy was in contact with the corrosion resistant composition.